In the manufacture of panels for use as air frame components in aircraft, space vehicles, missiles and the like it is commonly required that a large number of high precision holes be cut in a panel. The holes usually receive rivets, bolts or other fasteners to be used in fabrication. Oftentimes hundreds or even thousands of holes are cut in a workpiece at a given station. In order to form such holes, it is conventional practice to use high precision positive feed drilling devices which can be used to cut the holes rapidly, precisely and to a desired depth. Such drilling machines normally take the form of a drill head incorporating a positive feed threaded spindle which is used to both rotate and work and advance or retract a tool holder such as an arbor in which is secured a cutting tool such as a drill bit or reamer.
In order to dissipate heat and also remove cuttings from the work face at the confluence of the cutting tool and the workpiece, e.g. at the bottom of a hole being drilled by a drill bit, it is a conventional practice to employ a circulating fluid in conjunction with the cutting operation. The circulating fluid may be a gas or liquid and it may be applied to the work site through an attendant auxiliary nozzle or it may be applied to the work site through a supply passage extending through the cutting tool.
As noted previously, either gaseous or liquid circulating fluids can be used. In most highly repetitive cutting applications such as in the drilling of holes in air frame components, it has been the general practice to use a liquid circulating fluid. The use of a liquid provides for good cooling of the bit by heat transfer to the liquid medium as well as good lubrication. However, gasses have been used in some cutting operations and it has also been proposed use a gaseous cutting fluid in which a lubricant is incorporated into the gas to form a mist.
U.S. Pat. Nos. 3,478,843 to Eckhardt and 3,577,808 to Visser et al. disclose the supply of a mist of a coolant and lubricant under applied air pressure to a work site through a nozzle. For example, the patent to Visser discloses supplying a cutting fluid, a mixture of a lubricant and a coolant or refrigerant, e.g. "Freon", into a pressurized air stream which directed by a nozzle against the tip of a drill bit as it penetrates a workpiece. U.S. Pat. No. 3,605,551 to Steward discloses the use of an evaporative coolant such as alcohol, carbon tetrachloride, trichloroethylene, or trichlorotrifluorethane which is added to a commercially available cutting solution. The mixture is then applied under air pressure as a super coolant vapor mist to the confluence of the cutting tool and workpiece. Steward discloses high drilling speeds of 100 rpm.
U.S. Pat. No. 4,573,836 to Andersson discloses a rotary drill system in which the cutting tool is provided with an internal passage to which circulating fluid is supplied via a rotary swivel. The internal passageway terminates in two openings adjacent carbide cutting inserts at the end of the cutting tool. U.S. Pat. No. 4,640,652 discloses a cutting system in which a tap is provided with a central passageway for the circulation of coolant fluid.
The use of inert gases is disclosed in Japanese Patent 58-22609 and U.S. Pat. No. 4,563,924 to Runkle et al. In the Japanese patent, nitrogen gas under a pressure of 5 kg/cm.sup.2 (about 70 psi) is supplied to a core cutter. The core cutter is said to be cooled to a temperature of -20.degree. to +20.degree. C. The patent to Runkle et al discloses a procedure for reducing the oxygen concentration at the confluence of a nonoxide ceramic cutting tool and a workpiece by supplying concentric annular curtains of inert gas to the work face. The oxygen concentration in the work zone is reduced to 10% or less of ambient oxygen.